In recent years, along with a tendency to high brightness and whitening of a light emitting device having a light emitting element such as a light emitting diode, a light emitting device using a light emitting element has been used for a backlight of a mobile phone, a liquid crystal TV, etc. In such a light-emitting device, a substrate is required to have a high thermal conductivity, be able to quickly dissipate heat generated from the light-emitting element and have a high reflectance and excellent productivity.
In order to satisfy such requirements, it has been studied to use a glass ceramic substrate as a substrate for a light emitting device. The glass ceramic substrate comprises a glass powder and a ceramic powder such as an alumina powder and has a large difference in refractive index between glass and ceramics and many interfaces between them. Thus, the glass ceramic substrate has a higher reflectance than conventional ceramic substrates. However, the glass ceramic substrate is required to have a further high reflectance to be used as a substrate for mounting a light emitting element.
Further, for reducing unevenness in various properties such as reflectance and strength, the glass ceramic substrate is also required to have a property to suppress shrinkage at the time of firing, namely firing shrinkage.
In order to increase the reflectance of a glass ceramic substrate, a method of incorporating ceramic particles having a refractive index higher than that of alumina particles, namely high refractive index particles, has been studied. However, the high refractive index particles tend to have a low sintering property. Accordingly, the content of the high refractive index particles cannot be made high, or the composition of glass is significantly restricted. Thus, the degree of freedom of the design is decreased. Accordingly, in order to make it possible to select the glass composition from a wide range, it is desired to increase the reflectance and reduce firing shrinkage by using alumina particles or the like having good sintering property with glass.
As a method for improving the reflectance, a method of limiting the particle size and the occupation area has been known (for example, Patent Document 1). On the other hand, as a method for suppressing the firing shrinkage, a method of using flat particles and aligning these flat particles in the designated direction has been known (for example, Patent Document 2). Further, as a method for improving the strength, a method of dispersing particles having an aspect ratio of at least 4 has been known (for example, Patent Document 3). Further, it has been known that regarding to the crystal structure, from the viewpoint of the strength, an α-alumina is preferred, and from the viewpoint of the dielectric constant, a γ-alumina is preferred (for example, Patent Document 4).